Redox Flow Batteries Based on Supporting Solutions Containing Chloride

ABSTRACT

Redox flow battery systems having a supporting solution that contains Cl −  ions can exhibit improved performance and characteristics. Furthermore, a supporting solution having mixed SO 4   2−  and Cl −  ions can provide increased energy density and improved stability and solubility of one or more of the ionic species in the catholyte and/or anolyte. According to one example, a vanadium-based redox flow battery system is characterized by an anolyte having V 2+  and V 3+  in a supporting solution and a catholyte having V 4+  and V 5+  in a supporting solution. The supporting solution can contain Cl −  ions or a mixture of SO 4   2−  and Cl −  ions.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under ContractDE-AC0576RL01830 awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

BACKGROUND

A redox flow battery (RFB) stores electrical energy in reduced andoxidized species dissolved in two separate electrolyte solutions. Theanolyte and the catholyte circulate through a cell electrode separatedby a porous membrane. Redox flow batteries are advantageous for energystorage because they arc capable of tolerating fluctuating powersupplies, repetitive charge/discharge cycles at maximum rates,overcharging, overdischarging, and because cycling can be initiated atany state of charge.

However, among the many redox couples upon which redox flow batteriesare based, a number of disadvantages exist. For example, many systemsutilize redox species that are unstable, are highly oxidative, aredifficult to reduce or oxidize, precipitate out of solution, and/orgenerate volatile gases. In many ways, the existing approaches toaddressing these disadvantages have been ad hoc and can include theimposition of restrictive operating conditions, the use of expensivemembranes, the inclusion of catalysts on the electrodes, and/or theaddition of external heat management devices. These approaches cansignificantly increase the complexity and the cost of the total system.Therefore, a need for improved redox flow battery systems exists.

SUMMARY

The present invention includes redox flow battery systems having asupporting solution that comprises Cl⁻ anions. In one embodiment, avanadium-based redox flow battery system is characterized by an anolytecomprising V²⁺ and V³⁺ in a supporting solution and a catholytecomprising V⁴⁺ and V⁵⁺ in a supporting solution. The supporting solutioncan comprise Cl⁻ ions or a mixture of SO₄ ²⁺ and Cl⁻ ions. The use ofCl⁻ ions can improve the energy density and the stability of anall-vanadium battery compared to the traditional use of SO₄ ²⁻ ions.

Supporting solutions comprising both SO₄ ²⁻ and Cl⁻ ions can furtherimprove the performance and characteristics by all-vanadium batteries byincreasing the solubility of the vanadium cations as described ingreater detail below. In particular embodiments, the concentration ratioof Cl⁻ to SO₄ ²⁻ can be between 1:100 and 100:1. In other embodiments,the ratio can be between 1:10 and 10:1. In still other embodiments, theratio can be between 1:3 and 3:1.

For all-vanadium batteries, the Cl⁻ in the supporting solution canimprove stability of the vanadium cations. For example, in traditionalflow redox batteries, V⁵⁺ can tend to form V₂O₅ at temperatures above40° C. However, the presence of Cl⁻ ions in the supporting solution canresult in the formation of VO₂Cl(H₂O)², a stable, neutral species.Accordingly, embodiments of the present invention can operate at celltemperatures greater than 40° C. Preferably, the cell temperature duringoperation is between −35° C. and 60° C. Furthermore, the embodiments ofthe present invention can operate without thermal management devicesactively regulating the cell temperature. In conventional all-vanadiumflow redox batteries, thermal management devices are required tomaintain the battery below the cell temperature at which the V cationscome out of solution.

Further still, vanadium cation concentrations in batteries of thepresent invention can exceed those of traditional SO₄ ²⁻-basedbatteries. In some embodiments, the vanadium cation concentration isgreater than 0.5M. In others, the vanadium cation concentration isgreater than 1.7M. In still others, the vanadium cation concentration isgreater than 2.5M.

In a preferred embodiment, the state of charge condition is greater than0% and less than 100% during operation. In other words, the batteriesare preferably not operated to full charge or discharge states.

In another embodiment of the present invention, a redox flow batteryhaving a supporting solution comprising Cl⁻ ions comprises an anolytecomprising V²⁺ and V³⁺ in the supporting solution, a catholytecomprising Fe²⁺ and Fe³⁺ in the supporting solution, and a membraneseparating the anolyte and the catholyte. The anolyte and catholyte cancomprise V cations and Fe cations, respectively, or the anolyte andcatholyte can each contain both V and Fe cations in a mixture. In someinstances, the concentrations of the Fe cations and/or the V cations canbe greater than 0.5M.

Relative to some highly oxidative redox couples, The Fe and V couple isless aggressive. Accordingly, expensive oxidation-resistant membranessuch as sulfonated tetrafluoroethylene based fluoropolymer-copolymersare not necessary. On a cost basis, other less expensive options can bepreferable. Accordingly, some embodiments of the Fe/V battery systemcomprise hydrocarbon-based membranes or micro-porous separators. Oneexample of a hydrocarbon membrane includes, but is not limited to asulfonated poly(phenylsulfone) membrane. Other ion exchange membranescan be suitable.

In another embodiment, the Fe/V battery system comprises electrodes,which do not contain a redox catalyst, in contact with the anolyte andthe catholyte. Redox catalysts arc sometimes necessary for species thatare difficult to reduce and/or oxidize and can include metals or metaloxides. Redox catalysts are preferably absent from the electrodes usedin embodiments of the present invention.

Some embodiments of the Fe/V battery system operate at cell temperaturesbelow 60° C. In other embodiments, the system operates at celltemperatures between −20° C. and 50° C. In preferred embodiments, thesystem does not include a heat management device actively regulating thecell temperature. In particular, no heat management device is utilizedto heat the Fe/V battery system.

In the Fe/V battery systems, supporting solutions comprising both SO4²⁻and Cl⁻ ions can further improve the performance and characteristics byincreasing the solubility of the cations as described in greater detailbelow. In particular embodiments, the concentration ratio of Cl⁻ to SO₄²⁻ can be between 1:100 and 100:1. In other embodiments, the ratio canbe between 1:10 and 10:1. In still other embodiments, the ratio can bebetween 1:3 and 3:1. Instances in which the supporting solutioncomprises both Cl⁻ to SO₄ ²⁻ and the anolyte and catholyte both compriseV and Fe cations, the concentration of V²⁺, V³⁺, Fe²⁺, and Fe³⁺ aregreater than 1.5M. Instances in which the anolyte comprises V cationsand the catholyte comprises Fe cations, the concentrations of V²⁺ andV³⁺ are greater than 2M in the anolyte and concentrations of Fe²⁺ andFe³⁺ are greater than 2M in the catholyte.

In yet another embodiment, a redox flow battery system comprises asupporting solution that comprises a mixture of SO₄ ²⁻ and Cl⁻. Asdescribed elsewhere herein, a supporting solution having mixed SO₄ ²⁻and Cl⁻ can provide increased energy density and improved stability andsolubility of one or more of the ionic species in the catholyte and/oranolyte, such as Fe²⁺, Fe³⁺, Cr²⁺, Cr³⁺, and others. In particularembodiments, the concentration ratio of Cl⁻ to SO₄ ²⁻ can be between1:100 and 100:1. In other embodiments, the ratio can be between 1:10 and10:1. In still other embodiments, the ratio can be between 1:3 and 3:1.In still other embodiments, other halogen ions can be mixed with SO₄ ²⁻,including but not limited to, F⁻.

The purpose of the foregoing abstract is to enable the United StatesPatent and Trademark Office and the public generally, especially thescientists, engineers, and practitioners in the art who are not familiarwith patent or legal terms or phraseology, to determine quickly from acursory inspection the nature and essence of the technical disclosure ofthe application. The abstract is neither intended to define theinvention of the application, which is measured by the claims, nor is itintended to be limiting as to the scope of the invention in any way.

Various advantages and novel features of the present invention aredescribed herein and will become further readily apparent to thoseskilled in this art from the following detailed description. In thepreceding and following descriptions, the various embodiments, includingthe preferred embodiments, have been shown and described. Includedherein is a description of the best mode contemplated for carrying outthe invention. As will be realized, the invention is capable ofmodification in various respects without departing from the invention.Accordingly, the drawings and description of the preferred embodimentsset forth hereafter are to he regarded as illustrative in nature, andnot as restrictive.

DESCRIPTION OF DRAWINGS

Embodiments of the invention are described below with reference to thefollowing accompanying drawings.

FIG. 1 is a graph of current versus voltage comparing all-vanadium RFBsusing chloride-containing and sulfate-containing supporting solutions.

FIG. 2 compares thermodynamic equilibrium concentrations (a) andequilibrium potentials (b) of chlorine and oxygen gases in vanadiumchloride RFB systems.

FIG. 3 compares cyclic performances of vanadium chloride RFB systems andvanadium sulfate RFB systems.

FIG. 4 compares cyclic voltammetry curves of a vanadium-chloride-sulfatesolution and a vanadium sulfate solution.

FIG. 5 is a graph of equilibrium concentrations of chlorine in thepositive side of a vanadium-chloride-sulfate cell at various states ofcharge.

FIG. 6 is a diagram depicting structures of VO₂ ⁻ in sulfuric acid (a)and in hydrochloric acid (b).

FIG. 7 is a graph of cyclic coulombic efficiency, voltage efficiency,and energy efficiency for a vanadium-chloride-sulfate RFB system.

FIG. 8 are cyclic voltammetry curves in a Fe/V and Cl-containingsolution using two different electrodes.

FIG. 9 contains graphs demonstrating the electrochemical performance ofan Fe/V redox flow cell using a Cl-containing supporting solution.

FIG. 10 shows cyclic Coulombic efficiency, voltage efficiency, andenergy efficiency (a) as well as cell charge capacity and charge energydensity change (b) for a Fe/V cell employing S-Radel as membrane

DETAILED DESCRIPTION

The following description includes the preferred best mode as well asother embodiments of the present invention. It will be clear from thisdescription of the invention that the invention is not limited to theseillustrated embodiments but that the invention also includes a varietyof modifications and embodiments thereto. Therefore the presentdescription should he seen as illustrative and not limiting. While theinvention is susceptible of various modifications and alternativeconstructions, it should be understood, that there is no intention tolimit the invention to the specific form disclosed, but, on thecontrary, the invention is to cover all modifications, alternativeconstructions, and equivalents falling within the spirit and scope ofthe invention as defined in the claims.

FIGS. 1-10 show a variety of embodiments and aspects of the presentinvention. Referring first to FIG. 1, current versus voltage data isplotted for vanadium ion redox couples using either chloride or sulfatesupporting solutions. Three redox couples were observed in the chloridesystem, indicating that two redox couples (VO²⁺/VO₂ ⁺ for the positiveand V²⁺/V³⁺ for the negative) can be employed for a redox flow battery.Electrochemical reversibility of the V⁴⁺/V⁵⁺ couple was similar to thatof a sulfate system, but that of the V²⁺/V³⁺ was significantly improvedin the chloride system. For example, the peak potential difference is0.351 V in the sulfate system and 0.509 V in the chloride system.

According to quantum chemistry calculations, the V⁵⁺ species in thechloride solution forms VO₂Cl(H₂O)₂, which is a more stable neutralspecies than [VO₂(H₂O)₃]⁺, the species commonly formed in the sulfatesolution. However, V²⁺, V³⁺ and V⁴⁺ in the chloride solution have asimilar structure to that in the sulfate solution. Based on the above,the half cell reaction shown in Eq. (2) for the positive pole describeswell the electrochemistry. The standard potential of this half cellreaction is expected to be slightly higher than that of the conventionalsulfate system resulting from a different V⁵⁺ species. By forming thisnew structure, the thermal stability of the V⁵⁺ in the chloride solutionwas significantly improved.

In the chloride system, oxygen and chlorine gas evolution duringcharging can reduce columbic efficiency. Referring to FIG. 2( a),equilibrium concentrations of chlorine or oxygen estimated fromthermodynamic equilibrium for Eq. (1) and (4), and Eq. (1) and (5),respectively, are shown as a function at the state of charge (SOC) atvarious temperatures. It should be noted that hypochlorite can benegligible because the equilibrium constant of Eq. (6) is 6.35E-13 at25° C. The actual concentrations of the chlorine should be lower thanthe values depicted in FIG. 2( a) due to complex formation. Within atypical operation window of redox flow batteries (i.e., SOC of 20˜80%),the chlorine concentration is negligible even at 40° C. However, gasevolution may be significant at SOC values approaching 100%.

Chlorine has much higher solubility in water than oxygen; Henry'sconstant of chlorine and oxygen in water at 25° C. is 0.062 mol/L-atmand 0.0013 mol/L-atm, respectively. Assuming partial pressure of oxygenand chlorine is 0.1 bar, the equilibrium potential of Eq. (4) and (5)was calculated for 2.3 M V in 10 M total chloride system, and is shownin FIG. 2( b) as a function of SOC. Based on the data, VO₂ ⁺ isthermodynamically stable from oxygen evolution below an 80% SOC, andfrom chlorine evolution below a 98% SOC. To maintain saturation ofchlorine in the electrolyte solution, the flow battery is preferablyoperated in a closed system. A closed system is also advantageous toprevent rapid oxidation of V²⁺ and V³⁺ by air and to minimizeelectrolyte loss.

2Cl⁻⇄Cl₂+2e E°=1.36V vs. NHE   (4)

2H₂O⇄O₂+4H⁺+4e E°=1.23V vs. NHE   (5)

Cl₂+H₂O⇄2H⁺+Cl⁻+ClO⁻  (6)

In addition to thermodynamic equilibrium, electrode overpotential cancontribute to gas evolution. The equilibrium potential of reaction (4)is higher than that of reaction (5), but oxygen evolution can benegligible compared to chlorine evolution because of a higheroverpotential on the electrode. For example, the chlorine evolutionoverpotential on a graphite porous electrode was 0.12 V at 25° C. atcharge current of 22 mA/cm² for a Zn/Cl₂ battery (see N. Watanabe, T.Touhara, New Mat. New Processes, 1 (1981) 62). This overpotential washigher than that of the oxidation reaction in Eq. (2) above. Therefore,the chlorine evolution reaction can be negligible except for an SOC of˜100%. Because the electrode overpotential of chlorine evolutiondecreases with increasing temperature, charging is preferably controlledbelow SOC of 90˜95% to prevent chlorine evolution, especially atelevated temperature.

The thermal stabilities of different vanadium ion species in eithersulfate or chloride supporting solutions are summarized in Table 1. Inthe sulfate system, with more than 1.7 M vanadium, V²⁺ and V⁴⁺experienced precipitation at low temperatures (−5° C. and 25° C.), andV⁵⁺ suffered from precipitation at 40° C. In the chloride system,thermal stability was significantly improved, V²⁺, V⁴⁺ and V⁵⁺ werestable for more than 10 days in the temperature ranges of −5 and 50° C.for 2.3 M vanadium. According to nuclear magnetic resonance data (notshown). V⁵⁺ in the sulfate solution exists as a form of [VO₂(H₂O)₃]⁺.With increasing temperature, this complex decomposed into VO(OH)₃ andH₃O⁺, and then VO(OH)₃ is converted into a precipitate of V₂O₅.3H₂O. Asmentioned elsewhere herein, V⁵⁺ is believed to exist as a stable neutralform of VO₂Cl(H₂O)₂ in the chloride solution. Regardless, the supportingsolutions comprising Cl⁻ can enable better stability at highertemperature.

TABLE 1 Comparison of thermal stability of V^(n+) for chloride andsulfate systems. V^(n+) V^(n+) H⁺ SO₄ ²⁻ Cl⁻ T Time for species [M] [M][M] [M] (° C.) precipitation V²⁺ 2 6 5 0 −5 419 hr  2 6 5 0 25 Stable(>20 d) 2 6 5 0 40 Stable (>20 d) V³⁺ 2 4 5 0 −5 Stable (>20 d) 2 4 5 025 Stable (>20 d) 2 4 5 0 40 Stable (>20 d) V⁴⁺ (VO²⁺) 2 6 5 0 −5 18 hr2 6 5 0 25 95 hr 2 6 5 0 40 Stable (>20 d) V⁵⁺ (VO₂ ⁺) 2 8 5 0 −5 Stable(>20 d) 2 8 5 0 25 Stable (>20 d) 2 8 5 0 40 95 hr 1.8 8.4 5 0 40 358hr  V²⁺ 2.3 5.4 0 10 −5 Stable (>20 d) 2.3 5.4 0 10 25 Stable (>20 d)2.3 5.4 0 10 40 Stable (>20 d) V³⁺ 1.5 3.0 0 7.5 −5 Stable (>20 d) 1.83.0 0 8.4 −5 124 hr  2.3 3.1 0 10 −5 96 hr 2.3 3.1 0 10 25 Stable (>20d) 2.3 3.1 0 10 40 Stable (>20 d) V⁴⁺ (VO²⁺) 2.3 5.4 0 10 −5 Stable (>20d) 2.3 5.4 0 10 25 Stable (>20 d) 2.3 5.4 0 10 40 Stable (>20 d) V⁵⁺(VO₂ ⁺) 2.3 7.7 0 10 −5 Stable (>20 d) 2.3 7.7 0 10 25 Stable (>20 d)2.3 7.7 0 10 40 Stable (>20 d) 2.3 7.7 0 10 50 Stable (>10 d)

When operation of an all Cl⁻ system occurs at, or below, freezingtemperatures (i.e., 0° C.), the tank containing the electrolyte ispreferably insulated to maintain waste heat from the flow battery, whichcan be approximately 20% of total energy. Operation above the freezingtemperature, energy density can be improved by approximately 35% owingto higher vanadium concentration compared to the sulfate system.Stabilization of the V³⁺ species at the lower temperature can beachieved by using a supporting solution containing both SO₄ ²⁻ and Cl⁻,as is described in greater detail elsewhere herein.

Typical energy efficiency of vanadium redox flow batteries is about 80%;indicating 20% of the energy is released as waste heat during eachcycle. Assuming an adiabatic system, the electrolyte temperature canincrease by about 4° C. per cycle. The thermal stability of electrolytesat higher temperatures can be a major concern, especially on hot days.For conventional all vanadium sulfate systems, active thermal managementdevices such as heat exchangers are commonly employed to maintain thecell temperature below 40° C. and to prevent precipitation of V⁵⁺. Anactive thermal management system is not preferable and is a significantparasitic energy loss. Embodiments of the present invention based onvanadium and Cl-containing supporting solution can be operated at a widerange of temperatures between 0 to 50° C. without an active thermalmanagement system, improving significant system efficiency and alsoreducing cost.

Flow cell performance for different chloride and sulfate systems wereevaluated under the identical test conditions. The results at differentdischarging current densities were summarized in Table 2. Energy densityof the chloride system was ˜38 Wh/L, 30% higher than that of the sulfatesystem, resulting from the higher solubility of vanadium in the chloridesolution. This higher energy density can reduce the system cost byreducing tank size and footprint. Columbic efficiency of the chloridesystem was 94˜97% under operation of SOC between 0 and 100% (notinclusive), comparable to that of the sulfate system, which was 95˜97%.

TABLE 2 Comparison of discharging rate capability for VSRFB (1.7M V in5M total sulphate) and VCRFB (2.3M V in 10M total chloride). EfficiencyEnergy density* CD Capacity (Ah) (Wh/L) Coulomb Energy Voltage (mA/cm²)Cl⁻ SO₄ ²⁻ Cl⁻ SO₄ ²⁻ Cl⁻ SO₄ ²⁻ Cl⁻ SO₄ ²⁻ Cl⁻ SO₄ ²⁻ 100 2.75 2.1435.5 27.9 0.94 0.95 0.80 0.83 0.85 0.87 75 2.75 2.14 36.6 28.4 0.96 0.960.84 0.85 0.87 0.89 50 2.75 2.14 37.8 29.1 0.97 0.96 0.87 0.88 0.90 0.9125 2.74 2.13 38.7 29.7 0.97 0.97 0.90 0.91 0.92 0.94 *Note that energydensity was calculated only by electrolyte volume.

Cyclic performance of both systems at ambient temperature was alsoevaluated by cycling between 1.6V and 1.2V, which are shown in FIG. 3.The capacities of both systems slightly decreased with cycles because ofdifferent transport rate of vanadium species across the membrane. Thiscapacity loss can be recovered by remixing and rebalancing anolyte andcatholyte because a single element of V is used for both solutions.Energy and coulombic efficiencies for the chloride system was stablewith cycles and comparable to those of sulfate system. It can beconcluded that the novel all vanadium chloride flow battery can bestably operated in a comparable energy efficiency to the sulfate system,while delivering energy density of ˜38 Wh/L, 30% higher than the sulfatesystem. Chlorine evolution or V⁵⁺ electrolyte stability in the chloridesolution was not an issue under closed operation conditions.

Electrolyte for the all vanadium chloride systems described above wasprepared by dissolving V₂O₃ in concentrated HCl (38%). The electrolytefor the all vanadium sulphate system was fabricated by dissolvingVOSO₄.3.8 H₂O in sulfuric acid (95.8%).

Cyclic voltammetry (CV) tests for the chloride system were conductedwith identical graphite felts (φ=5 mm mm) used in flow cell testing toidentify redox couples and electrochemical reversibility using Solartron1287 potentiostat. The scan rate was 0.5 mV/s.

Cell performance of two different systems was measured using a flow cellsystem under identical test conditions. The apparent area of thegraphite felt was 10 cm² (2 cm×5 cm), in contact with NAFION 117membrane, a sulfonated tetrafluoroethylene basedfluoropolymer-copolymer. Other proton-exchange membranes can besuitable. 2.3 M vanadium in 10 M total chloride solution and 1.7 M V in5 M total sulphate solution were used for performance comparison. Eachelectrolyte volume and flow rate was 50 mL and 20 mL/min, respectively.The effect of different discharging current densities was evaluated inthe first 5 cycles with the same charging current of 50 mA/cm². The flowcell was charged to 1.7 V and then discharged to 0.8 V. After that, theflow cell was cycled between 1.6 V and 1.2 V at 50 mA/cm².

The electrolyte stability tests were carried out in polypropylene tubesat −5, 25, 40, and 50° C., using about 5 ml solution for each sample.During the stability tests, the samples were kept static without anyagitation, and were monitored daily by naked eye for the formation ofprecipitation.

Referring to Table 3, which summarizes the stability of V²⁺, V³⁺, V⁴⁺,and V⁵⁺ in sulfuric acid solutions, conventional sulfuric acid-onlyvanadium redox flow batteries (VRFB) can typically only be operated atcell temperatures between 10° C. and 40° C. with vanadium concentrationin the electrolytes less than 1.7M (with an energy density <25 Wh/L).The electrochemical reactions of an all vanadium sulfate redox flowbattery are represented by the following equations.

TABLE 3 Stability of V^(n+) cations in H₂SO₄ solution V^(n+) specieV^(n+), M H⁺, M SO₄ ²⁻, M T, ° C. Time for precipitation V²⁺ 2 6 5 −5Stable (>10 d) 2 6 5 25 Stable (>10 d) 2 6 5 40 Stable (>10 d) V³⁺ 2 4 5−5 Stable (>10 d) 2 4 5 25 Stable (>10 d) 2 4 5 40 Stable (>10 d) V⁴⁺ 26 5 −5 18 hr (VO²⁺) 2 6 5 25 95 hr 2 6 5 40 Stable (>10 d) V⁵⁺ 2 8 5 −5Stable (>10 d) (VO²⁺) 2 8 5 25 Stable (>10 d) 2 8 5 40 95 hr

As mentioned earlier, since the standard potential of reaction2Cl⁻=2e=Cl₂ (g) (E°=1.36 V) is much higher than that of Reaction (7),the supporting solution in a VRFB system can comprise Cl⁻ either as aSO₄ ²⁻ and Cl⁻ mixture or comprising Cl⁻ as the only anion. Moreover, asis described elsewhere herein, the use of mixed SO₄ ²⁻ and Cl⁻ in thesupporting solution is not limited to vanadium-based redox flowbatteries. Chloride and sulfate ions in the supporting solution can helpstabilize relatively higher concentrations of other cations as well.

FIG. 4 shows the cyclic voltammetry curve of a solution containing 2.5 MVOSO₄ and 6 M HCl. This curve is similar to that of a solutioncontaining 1.5 M VOSO₄ and 3.5 M H₂SO₄. Referring to FIG. 5, theequilibrium concentrations of Cl₂ gas in a vanadium sulfate-chloridecatholyte solution (containing 2.5 M vanadium, 2.5 M sulfate, and 6 Mchloride) under different state-of-charge (SOC) conditions werecalculated according to Reaction 12. Under normal flow battery operationconditions (i.e. T<40° C. and SOC<80%), the equilibrium concentration ofCl₂ gas is less than 10 ppm. Due to its high solubility in water (0.57 gCl₂ per 100 g water at 30° C.), most of the Cl₂ gas generated should bedissolved in the catholyte solutions. At high temperatures, SOC valueshigher than 80% are preferably avoided to minimize the Cl₂ gasevolution. Nevertheless, a closed system can be used to minimizecontinuous Cl₂ gas generation and to prevent Cl₂ gas emission to theenvironment. Such closed systems are commonly required for theconventional vanadium sulfate flow battery system to prevent oxidationof V²⁺ and V³⁺ by O₂ in air, and to prevent water loss from electrolytesolutions.

2VO₂ ⁺(a)+4H⁺(a)+2Cl⁻(a)=2VO²⁺(a)+Cl₂ (g)+2H₂O   (12)

The stability of different V^(n+) cations in Cl-containing solutions wasevaluated at a temperature range of −5° C., to 40° C. The results arcgiven in Table 4. More than 2.3 M VOCl₂ and VO₂Cl were stabilized in ˜6M HCl solution over a temperature range of −5° C. to 40° C., which ismuch higher than those in the sulfuric acid solution (˜1.5 M vanadium)over the same temperature range. The Cl⁻ anions appears to stabilize VO₂⁺ and VO²⁺ cations in the solution. Similar to that in the H₂SO₄solution, more than 2.3 M V²⁺ was also stabilized in ˜6 M HCl solutionat −5° C. to 40° C. However, compared to that in the H₂SO₄ solution, thestability of V³⁺ in HCl solution was decreased. At −5° C., only about1.5 M V³⁺ could be stabilized in 3 M HCl, whereas more than 2 M V³⁺ wasstabilized in 2 M H₂SO₄ (see Table 4).

TABLE 4 Stability of V^(n+) cations in HCl solution V^(n+) specieV^(n+), M H⁺, M Cl⁻, M T, ° C. Time for precipitation V²⁺ 2.3 5.4 10 −5Stable (>10 d) 2.3 5.4 10 25 Stable (>10 d) 2.3 5.4 10 40 Stable (>10 d)V³⁺ 1.5 3.0 7.5 −5 Stable (>10 d) 1.8 3.0 8.4 −5 124 hr 2.3 3.1 10 −5 96 hr 2.3 3.1 10 25 Stable (>10 d) 2.3 3.1 10 40 Stable (>10 d) V⁴⁺ 2.35.4 10 −5 Stable (>10 d) (VO²⁺) 2.3 5.4 10 25 Stable (>10 d) 2.3 5.4 1040 Stable (>10 d) V⁵⁺ 2.3 7.7 10 −5 Stable (>10 d) (VO₂ ⁺) 2.3 7.7 10 25Stable (>10 d) 2.3 7.7 10 40 Stable (>10 d)

Based on the stability test results above, Cl⁻ anions can helpstabilizing VO²⁺ and VO₂ ⁺ cations, and SO₄ ²⁻ anions can help stabilizeV³⁺ cations. Both Cl⁻ and SO₄ ²⁻ anions can stabilize V²⁺ cations.Accordingly, a sulfuric acid and hydrochloric acid mixture can stabilizehigh concentrations of all four vanadium cations. Table 5 gives thestability of different V^(n+) cations in two mixed SO₄ ²⁻ and Cl⁻solutions at −5° C. to 40° C. Without optimization, about 2.5 M of allfour V^(n+) cations were effectively stabilized in the 2.5 M SO₄ ²⁻-6 MCl⁻ mixed acid solution. At a higher vanadium concentration (3M), V²⁺,VO²⁺, and VO₂ ⁺ were also stabilized in the 3 M SO₄ ²⁻-6 M Cl⁻ mixedacid solution at −5° C. to 40° C. However, V³⁺ was only stable for about8 days at −5° C. Precipitation of VOCl was observed. Due to the largeamount of heat generation during the operation of a VRFB system, it isnot difficult to keep the cell temperature of the electrolytes higherthan −5° C. even when the ambient temperature is −5° C. or lower. Also,since a VRFB system is always operated under 80 to 90% state-of-chargeand state-of-discharge conditions, the highest concentration of V³⁺ in a3 M all vanadium flow battery system is 2.7 M (mixing with 0.3 M V²⁺, atthe end of 90% discharge) or 2.4 M (mixing with 0.6 M V²⁺, at the end of80% discharge). Therefore, in one embodiment, by using a sulfuric acidand hydrochloric acid mixture as the supporting solution, the VRFBsystem uses a supporting solution with a total vanadium concentrationhigher than 3 M.

TABLE 5 Stability of V^(n+) in the SO₄ ²⁻ + Cl⁻ solutions V^(n+) V^(n+)H⁺ SO₄ ²⁻ Cl⁻ T Time for specie [M] [M] [M] [M] (° C.) precipitation V²⁺3 6 3 6 −5 Stable (>10 d) 2.5 6 2.5 6 −5 Stable (>10 d) 2.5 6 2.5 6 25Stable (>10 d) 2.5 6 2.5 6 40 Stable (>10 d) 3 6 3 6 40 Stable (>10 d)V³⁺ 3 3 3 6 −5 192 hr (8 d) 2.5 3.5 2.5 6 −5 Stable (>10 d) 2.5 3.5 2.56 25 Stable (>10 d) 2.5 3.5 2.5 6 40 Stable (>10 d) 3 3 3 6 40 Stable(>10 d) V⁴⁺ (VO²⁺) 3 6 3 6 −5 Stable (>10 d) 2.5 6 2.5 6 −5 Stable (>10d) 2.5 6 2.5 6 25 Stable (>10 d) 2.5 6 2.5 6 40 Stable (>10 d) 3 6 3 640 Stable (>10 d) V⁵⁺ (VO₂ ⁺) 3 9 3 6 −5 Stable (>10 d) 2.5 8.5 2.5 6 −5Stable (>10 d) 2.5 8.5 2.5 6 25 Stable (>10 d) 2.5 8.5 2.5 6 40 Stable(>10 d) 3 9 3 6 40 Stable (>10 d) 2.7 V⁵⁺ + 7.7 3 6 50 Stable (>10 d)0.3 V⁴⁺ 2.7 V⁵⁺ + 7.7 3 6 60 Stable (>10 d) 0.3 V⁴⁺

At temperatures higher than 40° C. in traditional all-vanadium sulfateRFBs the stability of V⁵⁺ might decrease due to the formation of V₂O₅.However, as shown in Table 5, embodiments of the present invention usingmixed SO₄ ²⁻Cl⁻ solutions exhibit excellent stability with a mixture of2.7 M V⁵⁺ and 0.3 M V⁴⁺(corresponding to 90% of state-of-charge of a 3 MVRFB system) at temperatures as high as 60° C., indicating that Cl⁻anions can effectively stabilize the VO₂ ⁺ cations. As describedelsewhere herein, quantum chemistry calculations show that, inCl-containing solutions, a stable neutral species can form having theformula VO₂Cl(H₂O)₂. Referring to FIG. 6, a diagram depicts themolecular structure of [VO₂(H₂O)₃]⁺ and of VO₂Cl(H₂O)₂. In thisstructure, one Cl⁻ anion, two O²⁻ anions, and two H₂O molecules complexwith one V⁵⁺ in the first coordination shell. Without Cl⁻ anions in thesolution, two O²⁻ anions, and three H₂O molecules complex with V⁵⁺ inthe first coordination shell and a positively-charged specie with[VO₂(H₂O)₃]⁺ formula forms. Quantum chemistry calculations also indicatethat, at elevated temperatures, this positively charged species is proneto convert to V₂O₅-3H₂O by de-protonation (Reaction 13) and,condensation(Reaction 14). The structural differences appear to account for the muchimproved stability of VO₂ ⁺ cations in Cl⁻-containing solutions. Due tothe formation of stable VO₂Cl(H₂O)₂ structure, the equilibriumconcentration of Cl₂ gas in the catholyte solution should be lower thanthat shown in FIG. 5.

[VO₂(H₂O)₃]⁺→VO(OH)₃ +[H₃O]⁺  (13)

2VO(OH)₃→V₂O₅-3H₂O ↓  (14)

In embodiments comprising mixed SO₄ ²⁻Cl⁻ solutions, the stability ofV⁴⁺ is controlled by the solubility of VOSO₄, and the stability of V³⁺iscontrolled by the solubility of VOCl. The improvement of V⁴⁺ stabilityis due to the decrease of SO₄ ²⁻ concentration in the solution, and theimprovement of V³⁺ stability is due to the decrease of Cl⁻concentration. V²⁺ cation is stable in both Cl⁻ and SO₄ ²⁻-containingsolutions.

In traditional all-vanadium sulfate RFBs, energy efficiency is about80%, which means about 20% of the total energy is lost as waste heatduring each cycle. For an adiabatic system, this heat can raise thetemperature of the whole system by about 5° C. Due to the large amountof waste heat generation, stability of electrolytes at high temperaturerange is a major concern, especially during hot days. The embodiments ofthe present invention encompassing all-vanadium RFBs utilizing mixed SO₄²⁻Cl⁻ supporting solutions system can not only improve the energydensity, but can also expand the operation temperature window from10-40° C. to −5-60° C. During the cold winter days, limited insulationcan easily keep the temperature of the system above −5° C. Accordingly,in preferred embodiments, no active heat management is needed

Several small VRFB cells were used to evaluate the performances of twovanadium sulfate-chloride mixed systems (with 2.5 M and 3.0 M vanadium).For comparison, the performance of a vanadium sulfate system (with 1.6 Mvanadium) was also measured. The results are summarized in Table 6. Thesulfate-chloride mixed systems show much higher energy density than thesulfate system. Even with higher vanadium concentration, the allvanadium sulfate-chloride mixed systems still showed similar energyefficiency to that of the vanadium sulfate system. FIG. 7 provides thecyclic coulombic efficiency, voltage efficiency, and energy efficiencyof the 2.5 M all vanadium sulfate-chloride mixed acid system atdifferent ambient temperatures. Stable performance was observed withthis new system. During a course of 20 days of operation, the gas-phasepressures of the anolyte and catholyte containers remained constant,indicating no significant gas evolution occurred in the whole system.The viscosity and density of a solution containing 2.5 M VOSO₄ and 6 MHCl at 30° C. is 6.1 cP and 1.40 g/ml respectively, slightly lower thanthe 6.4 cP and 1.45 g/ml for a solution containing 2.0 M VOSO₄ and 3.0 MH₂SO₄.

TABLE 6 Performance of all vanadium redox flow cells using the mixed SO₄²⁻Cl⁻ supporting solutions Energy Density Coulombic Efficiency EnergyEfficiency Voltage Efficiency Current of Wh/L η_(C) η_(E) η_(V)Discharge, 2.5VS 3VS 1.6V 2.5VS 3VS 1.6V 2.5VS 3VS 1.6V 2.5VS 3VS 1.6VmA/cm² 6Cl 6Cl 4.5S 6Cl 6Cl 4.5S 6C1 6Cl 4.5S 6Cl 6Cl 4.5S 100 36.2 39.522.3 0.95 0.95 0.94 0.81 0.76 0.83 0.85 0.80 0.88 75 37.5 40.8 22.4 0.960.96 0.94 0.84 0.81 0.85 0.88 0.84 0.90 50 38.5 41.8 22.6 0.96 0.97 0.940.87 0.85 0.87 0.91 0.88 0.92 25 39.2 43.1 22.6 0.96 0.97 0.94 0.90 0.890.88 0.93 0.91 0.94 1. Cell operation conditions: 10 cm² flow cell,Charged to 1.7 V by 50 mA/cm² current. 2. 2.5VS 6HCl: 2.5M V 2.5M SO₄ ²⁻6M Cl⁻; 3VS6HCl: 3M V 3M SO₄ ²⁻ 6M Cl⁻: 1.6V 4.5S: 1.6M V 4.5M SO₄ ²⁻.

The experiment details related to the all-vanadium RFBs using mixed SO₄²⁻ Cl⁻ supporting solutions are as follows. The flow cells consisted oftwo graphite electrodes, two gold-coated copper current collectors, twoPTFE gaskets, and a Nafion® 117 membrane. The active area of theelectrode and the membrane was about 10 cm². An Arbin battery tester wasused to evaluate the performance of flow cells and to control thecharging and discharging of the electrolytes. A Solartron 1287potentiostat was employed for cyclic voltammetry (CV) experiments. Theflow rate was fixed at 20 mL/min, which was controlled by a peristalticpump. A balanced flow cell contained about 50 mL anolyte and 50 mLcatholyte.

For cell performance evaluation and electrolyte solution preparation,the cell was normally charged at a current density of 50 mA/cm² to 1.7 Vand discharged to 0.8 V with a current density of 25 to 100 mA/cm². Cellcycling tests were performed at 90% state-of-charge andstate-of-discharge at a fixed charging and discharging current densityof 50 mA/cm².

The electrolyte solutions of V²⁺, V³⁺, V⁴⁺ and V⁵⁺ used in this workwere prepared electrochemically in flow cells using VOSO₄ (from AlfaAesar) and VCl₃ as starting chemicals. VCl₃ solutions were prepared bydissolving V₂O₃ (from Alfa Aesar) in HCl solutions. The electrolytestability tests were carried out in polypropylene tubes at −5° C.,ambient temperature, 40° C., 50° C., and 60° C., using about 5 mlsolution for each sample. During the stability tests, the samples werekept static without any agitation, and were monitored daily by naked eyefor the formation of precipitation. Solution viscosity was measuredusing a Ubbelohde calibrated viscometer tube.

Thermodynamic calculations of reaction 2VO₂ ⁺(a)+4H⁺(a)+2Cl⁻(a)=2VO²⁺were carried out using HSC Chemistry® 6.1 program from Outotec ResearchOy. Quantum chemistry calculations were carried out using the AmsterdamDensity Functional (ADF) program.

Yet another embodiment of the present invention encompasses a redox flowbattery system based on the redox couple of Fe and V. In this system,the anolyte comprises V²⁺ and V³⁺ in the supporting solution while thecatholyte comprises Fe²⁺ and Fe³⁺ in the supporting solution. The redoxreactions and their standard potentials can be described as follows:

Fe²⁺-e⇄Fe³⁺ E°=0.77 V vs. NHE   (15)

V³⁺+e⇄V²⁺ E°=−0.25 V vs. NHE   (16)

V³⁺+Fe²⁺⇄V² +Fe³⁺ E°=1.02 V vs. NHE   (17)

The Fe/V system of the present invention can provide significantbenefits while circumventing some of the intrinsic issues ofconventional systems. For example, certain embodiments of the Fe/Vsystem do not use catalysts and/or high-temperature management systems,which add to the complexity and cost of the system. Moreover theevolution of H₂ gas during charging is minimized since the workingpotential of V²⁺/V³⁺ (−0.25 V) is considerably higher than that ofothers, such as Cr²⁺/Cr³⁺ (−0.41 V). In the catholyte, the Fe²⁺/Fe³⁺redox couple is electrochemically reversible and can be less oxidativethan other common ionic species, such as V⁴⁺/V⁵⁺, which can result inhigher stability at high temperatures while avoiding expensive,oxidation-resistant membrane materials, such as sulfonatedtetrafluoroethylene based fluoropolymer-copolymer.

In one example using mixed Fe and V reactant solutions, an electrolytefor Fe/V redox flow battery tests was,prepared by dissolving VCl₃ (99%)and FeCl2 (98%) in concentrated HCl (38%). Cyclic voltammetry (CV) wascarried out in Fe/V+HCl solutions with various concentrations toidentify redox couples and electrochemical reversibility using aSOLARTRON 1287 potentiostat (SOLARTRON ANALYTICAL USA). A Pt wire andAg/AgCl electrode were used as the counter and reference electrodes,respectively. Glassy carbon electrodes and graphite felt (φ=5.5 mm)sealed onto a metal current collector were used as the workingelectrodes. The scan rate was 0.5 mV/s. Identical graphite felts withoutredox catalysts were used in both CV and flow cell testing.

Cell performance was measured under constant current methods using aflow cell system. The apparent area of graphite felt was 10 cm² (2 cm×5cm), in contact with membrane. 1.25 M Fe/V in 2.3 M HCl solution and1.17 M Fe/V in 2.15 M HCl solution were used with either a sulfonatedtetrafluoroethylene based fluoropolymer-copolymer (i.e., NAFION) or alow-cost hydrocarbon membrane such as sulfonated poly(phenylsulfone)membrane (i.e., S-RADEL), respectively. Each electrolyte volume and flowrate was 50 mL and 20 mL/min. The flow cell was charged to 1.3 V andthen discharged to 0.5 V at a current density of 50 mA/cm².

The chemical stability of commercially available membranes wasdetermined by soaking them in 0.15 M Fe³⁺ and 7 M total chloridesolution at 40° C., and in 0.1 M V⁵⁺ and 5 M total sulfate solution forcomparison. During the stability tests, the samples were kept staticwithout any agitation, and were monitored daily by naked eye for changesof color indicating oxidation of the membrane.

FIGS. 8( a) and (b) show CV results of 1.5 M Fe and V in a 1 Mhydrochloric acid supporting solution using glassy carbon and graphitefelt electrode, respectively. The current density is normalized to thegeometrical area of the working electrode. Similar CV spectra wereobserved on both the glassy carbon and graphite felt working electrodewith the graphite felt electrode demonstrating higher over potential dueto the low conductivity. Two redox peaks were observed indicating tworedox reactions, Fe³⁺/Fe²⁺ for the positive and V²⁺/V³⁺ for thenegative. More importantly, no significant hydrogen evolution currentwas observed at potentials below the V³⁺ reduction peak, indicating thatno significant gas evolution occurred at the negative electrode duringthe charging process when employing a V²⁺/V³⁺ redox couple. Oxidationand reduction peaks appear in the forward and reverse scans on thepositive side, which revealed a reversible redox couple of Fe³⁺/Fe²⁺with a potential at approximately 0.5 V. Similarly, there is no anodiccurrent observed associated with evolution of the Cl₂ and/or O₂ gas.Thus, the Fe³⁺/Fe²⁺ and V³⁺/V²⁺ redox couples in chloride supportingsolution can be used in the negative and positive half cells accordingto embodiments of the present invention.

FIG. 3 shows the results of Fe/V flow cell testing with a NAFION 117membrane. A plot of cell voltage versus time is given in FIG. 3( a).FIG. 3( b) demonstrates the cell voltage profile with respect to thecell capacity and the cell SOC. The SOC is calculated against themaximum charge capacity. Referring to FIG. 3( b), the Fe/V redox flowcell can be charged and discharged to a SOC in the range of 0˜100%. Autilization ratio of close to 100% can be achieved. Up to 50 cycles, theFe/V cell demonstrated stable columbic efficiency of ˜97%, energyefficiency of ˜78%, and voltage efficiency of ˜80% as shown in FIG. 3(c). The Fe/V cell also demonstrated excellent capacity and energydensity retention capability as shown in FIG. 3( d) with 0.1% loss percycle in charge capacity over 50 cycles.

Commercially available, low-cost membranes, including a micro-porousseparator, can be used in place of relatively expensive NAFION (i.e.,sulfonated tetrafluoroethylene based fluoropolymer-copolymer) membranes.Suitable alternative membranes can include, but are not limited to,hydrocarbon-based commercially available ion-exchange membranes; forexample, SELEMION® anion exchange membrane (APS, from Asahi Glass,Japan), SELEMION® cation exchange membrane (CMV, from Asahi Glass,Japan), and sulfonated poly(phenylsufone) membrane (S-RADEL® (RADEL®from Solvay Advanced Polymers, USA), and micro-porous separatorstypically used in lithium battery, for example; CELGARD® micro-porousseparator (Celgard, USA).

The electrochemical performance of a Fe/V cell employing a S-RADELmembrane was then evaluated using identical test protocols to that ofNafion membrane. The cell performance data is shown in FIGS. 5( a) and(b). Similar Coulombic efficiency, voltage efficiency, and energyefficiency with that of Nafion membrane were obtained.

In a preferred embodiment, the energy density of Fe/V RFB systems can beimproved by using a supporting solution comprising SO₄ ²⁻Cl⁻ mixed ionsto increase the reactant concentration in the anolyte and catholyte.Referring to Table 7, the solubility of Fe²⁺ and Fe³⁺ ions is higher inH₂SO₄—HCl mixed acids than in hydrochloric acid.

TABLE 1 Stability of Fe^(n+) cations in the H₂SO₄—HCl mixed solutionsFe^(n+) Fe^(n+), H⁺, SO₄ ²⁻, Cl⁻, T. Time for specie M M M M ° C.precipitation Fe²⁺ 2 4 2 4 25 Stable (>6 d) Fe³⁺ 2 6 2 6 25 Stable (>6d)

While a number of embodiments of the present invention have been shownand described, it will be apparent to those skilled in the art that manychanges and modifications may be made without departing from theinvention in its broader aspects. The appended claims, therefore, areintended to cover all such changes and modifications as they fall withinthe true spirit and scope of the invention.

1. A vanadium-based redox flow battery system; the system characterizedby an anolyte comprising V²⁺ and V³⁺ in a supporting solution and acatholyte comprising V⁴⁺ and V⁵⁺ in a supporting solution, thesupporting solution comprising Cl.
 2. The system of claim 1, wherein thesupporting solution comprises a mixture of SO₄ ²⁻ and Cl⁻ ions.
 3. Thesystem of claim 2, wherein the Cl⁻ to SO₄ ²⁻ concentration ratio isbetween 1:100 and,100:1.
 4. The system of claim 2, wherein the Cl⁻ toSO₄ ²⁻ concentration ratio is between 1:10 and 10:1.
 5. The system ofclaim 2, wherein the Cl⁻ to SO₄ ²⁻ concentration ratio is between 1:3and 3:1.
 6. The system of claim 1, wherein the catholyte comprisesVO₂Cl(H₂O)₂.
 7. The system of claim 1 having a cell temperature greaterthan 40° C. during operation.
 8. The system of claim 1 having a celltemperature between −35° C. and 60° C. during operation.
 9. The systemof claim 8 absent a thermal management device actively regulating thecell temperature.
 10. The system of claim 1 having a state-of-chargecondition greater than 0% and less than 100% during operation.
 11. Thesystem of claim 1, wherein vanadium cation concentration is greater than0.5M
 12. The system of claim 1, wherein vanadium cation concentration isgreater than 1.7 M.
 13. The system of claim 1, wherein vanadium cationconcentration is greater than 2.5 M. 14-19. (canceled)